The present disclosure relates generally to turbine engines and, more specifically, to turbine engines including counter-rotating turbine and fan assemblies.
At least some known gas turbine engines, such as turbofans, include a fan, a core engine, and a power turbine. The core engine includes at least one compressor, a combustor, and a high-pressure turbine coupled together in a serial flow relationship. More specifically, the compressor and high-pressure turbine are coupled through a shaft to form a high-pressure rotor assembly. Air entering the core engine is mixed with fuel and ignited to form a high energy gas stream. The high energy gas stream flows through the high-pressure turbine to rotatably drive the high-pressure turbine such that the shaft rotatably drives the compressor. After being discharged from the high-pressure turbine, the gas stream continues to expand as it flows through a low-pressure turbine positioned aft of the high-pressure turbine. The low-pressure turbine includes a rotor assembly coupled to a drive shaft and a fan. The low-pressure turbine rotatably drives the fan through the drive shaft.
Many modern commercial turbofans are manufactured with increasingly large bypass ratios to facilitate improving engine efficiency. However, increasing the bypass ratio of the turbofan results in an increased fan size and fan tip speed, which must be controlled to maintain the efficiency of the turbofan. As such, the low-pressure turbine in such turbofans is generally manufactured with a large number of stages, which increases the weight of the turbine and facilitates reducing the fan tip speed. Moreover, at least some known turbofans include a speed-reducing gearbox coupled along the drive shaft between the low-pressure turbine and the fan, such that the fan tip speed (m/s) is decoupled from the speed (RPMs) of the low-pressure turbine. However, utilizing speed-reducing gearboxes generally increases the weight and the complexity of the turbofan, and introduces further challenges in maintaining engine efficiency.